right|thumb|A cylinder block of the Sudan stamp jubilee issue of 1948
right|thumb|A plate strip of three of the 2-cent US stamp of 1895 includes the BEP's name.
right|thumb|A US plate block from 1972. Note the addition of a number for each color. This increased the cost of collecting plate blocks. That, plus the format change, discouraged many collectors.
A plate block is a block of stamps from the edge of the sheet which shows the plate or cylinder from which the stamps were printed.
Background
The numbering of printing plates has long been a part of quality control in the printing process. That way, if someone at the printing plant notices a problem with the printing of a certain stamp, the plate number can be used to locate the proper plate or cylinder so the problem can be investigated.
In the United Kingdom
In the U.K. plate blocks are also commonly known as cylinder blocks and are widely collected as they still form an important part of the printing process for current British stamps.
British stamps have also included a system of "controls" which were first used in February 1884 and continued until 1947. The "control" consisted of a letter in the selvedge of the sheet and was introduced as an accounting method by the printers due to the very large number of low value definitive stamps they were printing.
In the United States
Until the late 1960s, United States stamps included two rows of stamps attached to one another in a block of four or more, with printing information, including the printing plate number, on attached margin paper. A number is used to identify one specific plate or cylinder used to print the stamps.
Then plate block collecting changed in the US due to the addition of up to eight multi-digit numbers which represented different colors used to print the stamps. The numbers were printed along the selvage. This meant collectors needed many more stamps to save a single plate block. This lasted for about ten years before the post office reverted to the traditional single number for most stamps. Now, new issues often begin with a number such as "11111," with each digit in a different color. Stamps printed in large quantities may have multiple plate numbers so the next plate combination might be identified as "22222" and so on.